Re: [PATCH 1/3] fs: Avoid leaving freed inode on dirty list

Jan Kara
2020-04-27T10:05:28Z

On Sat 25-04-20 14:42:03, Xiaoguang Wang wrote:
> hi,
>
> > evict() can race with writeback_sb_inodes() and so
> > list_empty(&inode->i_io_list) check can race with list_move() from
> > redirty_tail() possibly resulting in list_empty() returning false and
> ^^^^^^^^^^^^^^^
> returning true?
> if (!list_empty(&inode->i_io_list))
> inode_io_list_del(inode);
> so "!list_empty(&inode->i_io_list)" returns false, and will not remove
> inode for wb->b_dirty list.

Yeah, right. I'll fix the mistake in the changelog. Thanks for noticing.

> > thus we end up leaving freed inode in wb->b_dirty list leading to
> > use-after-free issues.
> >
> > Fix the problem by using list_empty_careful() check and add assert that
> > inode's i_io_list is empty in clear_inode() to catch the problem earlier
> > in the future.
> From list_empty_careful()'s comments, using list_empty_careful() without
> synchronization can only be safe if the only activity that can happen to the
> list entry is list_del_init(), but list_move() does not use list_del_init().
>
> static inline void list_move(struct list_head *list, struct list_head *head)
> {
> __list_del_entry(list);
> list_add(list, head);
> }
>
> So I wonder whether list_empty(&inode->i_io_list) check in evict() can
> race with list_move() from redirty_tail()?

list_empty() check can race with list_move() but I don't think the outcome
of the racy check can ever be that the list is empty... Thinking about it
again, I'm not sure how even the list_empty() check could give false
positive because during the list_move() sequence, I don't think head->next
== head is ever true. So maybe this patch isn't needed at all (except for
the added BUG_ON() which is useful).

Honza

> >
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> > fs/inode.c | 9 ++++++++-
> > 1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/inode.c b/fs/inode.c
> > index 93d9252a00ab..a73c8a7aa71a 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -534,6 +534,7 @@ void clear_inode(struct inode *inode)
> > BUG_ON(!(inode->i_state & I_FREEING));
> > BUG_ON(inode->i_state & I_CLEAR);
> > BUG_ON(!list_empty(&inode->i_wb_list));
> > + BUG_ON(!list_empty(&inode->i_io_list));
> > /* don't need i_lock here, no concurrent mods to i_state */
> > inode->i_state = I_FREEING | I_CLEAR;
> > }
> > @@ -559,7 +560,13 @@ static void evict(struct inode *inode)
> > BUG_ON(!(inode->i_state & I_FREEING));
> > BUG_ON(!list_empty(&inode->i_lru));
> > - if (!list_empty(&inode->i_io_list))
> > + /*
> > + * We are the only holder of the inode so it cannot be marked dirty.
> > + * Flusher thread won't start new writeback but there can be still e.g.
> > + * redirty_tail() running from writeback_sb_inodes(). So we have to be
> > + * careful to remove inode from dirty/io list in all the cases.
> > + */
> > + if (!list_empty_careful(&inode->i_io_list))
> > inode_io_list_del(inode);
> > inode_sb_list_del(inode);
> >
--
Jan Kara <jack@suse.com>
SUSE Labs, CR